X-rays ruled out a fracture Saturday, and Cain told reporters Sunday he was still sore but understood the decision of manager Bruce Bochy and the Giants training staff.
Bochy called Philadelphia Phillies manager Charlie Manuel on Saturday night and informed the NL's skipper that Cain was "doubtful." Earlier in the week, the Giants were disappointed when Pablo Sandoval (who finished second in the Final Vote) was left off the team.
Tied for second in NL wins (10) this season, Cain will still travel to St. Louis alongside teammate Tim Lincecum, a strong candidate to start Tuesday's Midsummer Classic on the mound.
"Sometimes, I think you have to take it out of the player's hands," Bochy said before his team's final game of the first half. "Because if Matty went there, he would want to pitch, he would try to pitch. It's the risk versus reward for us."
Bochy added that if Cain attempted to throw an inning Tuesday while not 100 percent, the pitcher might be tempted to alter his delivery in order to perform on the grand stage. The fact that the Giants are already down one starter -- Randy Johnson was placed on the 15-day disabled list with a strained throwing shoulder Monday -- helped seal Cain's fate, too.
"We cannot afford to lose him," Bochy said. "What's more important is your team. The All-Star Game, it's a great honor to go and play ... but, at the same time, you have to think about your club. And he agreed."
Following his shortest career start (1 2/3 innings) Saturday, Cain said former Giants All-Stars assured him he would still be able to enjoy the festivities.
"You feel a little left out," Cain said. "I was talking to a couple guys, [and they said] 'Even if guys came and they were hurt and couldn't play, they deserved to be there.'"
Andrew Pentis is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.